Revert "Disable GPU clip masks"

This reverts commit 2272ea2a4b.

Reason for revert: https://crbug.com/863817
- We are experiencing GPU process instability on Mac
- Crash is in GL driver
- The instability only happens with GPU raster enabled
- This is in the culprit range and affect GL calls made

Original change's description:
> Disable GPU clip masks
> 
> Bug: skia:
> Change-Id: I3abacfd17ea6247d0bdb2ab8a2584a88bf468dad
> Reviewed-on: https://skia-review.googlesource.com/140776
> Reviewed-by: Stephen White <senorblanco@chromium.org>
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Chris Dalton <csmartdalton@google.com>

TBR=bsalomon@google.com,robertphillips@google.com,senorblanco@chromium.org,csmartdalton@google.com,senorblanco@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: skia:
Change-Id: Ia6c0e6f5dcede2aff7b0c3c6e19d47c255bb3991
Reviewed-on: https://skia-review.googlesource.com/144660
Reviewed-by: Christopher Cameron <ccameron@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>

src/gpu/GrClipStackClip.cpp

@@ -148,10 +148,6 @@ bool GrClipStackClip::UseSWOnlyPath(GrContext* context,
                                     bool hasUserStencilSettings,
                                     const GrRenderTargetContext* renderTargetContext,
                                     const GrReducedClip& reducedClip) {
-    // TODO: right now it appears that GPU clip masks are strictly slower than software. We may
-    // want to revisit this assumption once we can test with render target sorting.
-    return true;
-
     // TODO: generalize this function so that when
     // a clip gets complex enough it can just be done in SW regardless
     // of whether it would invoke the GrSoftwarePathRenderer.